why alkoxides are strong base.and which one is more basic in nature R3-O-(3 degree)or R-O-(1 degree)

Dear student
because alkoxide ion is an alkyl group attached with O(negative) where alkyl gp. is an electron releasing gp. which will enhance the electron density on oxygen due to which there will be intensification of negative charge on oxygen and this alkoxide ion will become unstable and would want any proton to attack on it to deintensify its negative charge . as we know strong base are those which take away the protons very quickly and effectively therefore it is a strong base.
Alkoxides are strongbasesand, when R is not bulky, goodnucleophilesand good ligands.